Newcastle trainer Darren Smith has lost an appeal against a 15-year ban for a string of doping offences.

Smith was disqualified by Racing NSW stewards last month on 42 separate charges relating to positive tests to cobalt returned by horses in his care.

He appealed the decision to the Racing NSW Appeal Panel which on Wednesday dismissed Smith's objections against the stewards' verdict and the severity of the penalty.

Smith's long-running cobalt case began last year and relates to a three-month period between February and May, 2014.

During that time, 20 raceday samples and one non-raceday sample from 17 different Smith-trained horses returned positive tests to cobalt.

Smith's lawyer argued that cobalt was not listed as a prohibited substance under the Rules of Racing at the time the samples were taken.

It became a declared prohibited substance Australia-wide on January 1, 2015.

However, stewards and the Appeal Panel were satisfied that cobalt fell under Australian Rule 178B which effectively states that any substance capable of directly or indirectly causing an action of effect in a horse's system is prohibited.

In its findings, the Appeal Panel agreed that cobalt was a banned substance at all times under the Rules of Racing, and that in high doses it also posed a potential welfare risk to horses.

Smith's ban has been backdated to May, 2014 when he closed his training business.

He is unable to reapply for his trainers' licence until 2029.